1. Field of the Invention
The present invention is directed to a compact, simple and inexpensive RF (radio frequency) transmitter which can transmit a signal at a fundamental frequency at a high power level while at the same time reducing the power of signals transmitted at harmonics of the fundamental frequency.
2. Discussion of the Background
When utilizing an oscillator to generate a transmitting signal, transmitting signals are generated not only at the fundamental frequency F.sub.0 of the oscillator, but, although not intentional, are also generated at harmonics of the fundamental frequency F.sub.2, F.sub.3, F.sub.4. . . (F.sub.2 represents the first harmonic, F.sub.3 represents the second harmonic, etc.).
Prior to being able to utilize and market a consumer electronics transmitter, the transmitter must meet Federal Communications Commission (FCC) approval and standards. When transmitting at certain fundamental frequencies F.sub.0, FCC standards typically require that the power of the signal transmitted at the fundamental frequency F.sub.0, and at the harmonics of that fundamental frequency F.sub.0, fall below certain predetermined thresholds. Typically, the maximum power threshold requirements at the harmonics will decrease sharply. FIG. 1 is a graph showing the type of threshold limits that would typically be required by FCC regulations.
It is generally desirable to maximize the ratio of the power of the transmitted signal at its fundamental frequency F.sub.0 to the power of the harmonics, to thereby maximize its range of transmission. However, it is very difficult to maximize the power of the transmitted signal at its fundamental frequency F.sub.0 and at the same time maintain the power of the transmitted signal at its harmonics F.sub.2, F.sub.3 and F.sub.4 below the maximum allowable level set by the FCC standards.
Without any system to reduce the power level of signals transmitted at the harmonics of the fundamental frequency F.sub.0 the power levels of the transmitted signal would resemble that shown in FIG. 2. As shown in FIG. 2, the power of the signal transmitted at the fundamental frequency F.sub.0 is below the appropriate threshold but the power of the signals transmitted at the harmonics of the fundamental frequency exceed their appropriate thresholds (the power of the signals transmitted at the frequencies for F.sub.0, F.sub.2, F.sub.3 and F.sub.4 are shown by the bars at those locations on the axis).
One method of overcoming this problem is to employ a plurality of filter circuits in conjunction with the oscillating circuit. These filter circuits will then operate to filter out the signals transmitted at the harmonics F.sub.2, F.sub.3, F.sub.4, etc. However, this solution has a drawback in that these filter circuits are expensive, consume a large amount of space and thus result in an enlargement of the circuit as a whole.
An alternate solution to reduce the power levels of the transmitted signal at the harmonics F.sub.2, F.sub.3, F.sub.4, is to reduce the power of the transmitted signal as a whole. That is, by reducing the overall power of the transmitted system, the power in the signal at the fundamental frequency F.sub.0 and at the harmonics F.sub.2, F.sub.3, and F.sub.4 will all be reduced and will thus fall below the maximum allowable thresholds set by FCC standards. This solution is shown in FIG. 3 where it can be seen that the signals transmitted at harmonics F.sub.2, F.sub.3, and F.sub.4 are within acceptable FCC thresholds. However, a sacrifice is here made in that the signal transmitted at the fundamental frequency F.sub.0 is much weaker than is allowable. Thus, the obvious drawback of such a compromise is that the power level of the signal at the fundamental frequency F.sub.0 is also reduced thereby diminishing the range and signal-noise ratio of the transmission signal.